Currently, the development in power electronics technology requires electronic products to be smaller and lighter, and makes a higher requirement on efficiency and electromagnetic compatibility. In a power electronics device, a filter inductor, a capacitor and a transformer account for a large proportion, so taking measures to reduce the size and weight of those components is a major means for the concept of being smaller and lighter. Improving switching frequency can accordingly improve cut-off frequency of a filter, and thus comparatively smaller inductors and capacitors can be chosen to reduce the size and weight of a filter. Improving switching frequency can reduce the size and weight of a transformer similarly.
However, while switching frequency is improved, switching losses will be increased and the problems of inductive turn-off, capacitive turn-on and diodes reverse recovery will be aggravated, which will give rise to reduction of circuit efficiency and increase of electromagnetic interference. In response to these problems, soft switching technology appears and it can solve the problem of switching losses in a converter, and meanwhile, solve the problem of EMI (Electro Magnetic Interference) caused by hard switching. Soft switching technology usually refers to ZVS (Zero Voltage Switch) and ZCS (Zero Current Switch), or similar ZVS and ZCS.
Soft switching converter of a DC-DC converter comprises: a plurality of soft switching technologies, such as, Resonant Converter, Quasi Resonant Converter, Multiple Resonant Converter, Zero Switching PWM Converter and Zero Transition PWM Converter and the like.
A Resonant Converter, actually a DC switch power supply loading a resonant converter, is obtained by the method of simply adding resonant elements to a standard PWM Converter. Resonant converts can be categorized into series resonant converter and parallel resonant converter in terms of resonant modes of resonant elements, and can be categorized into series load resonant converter and parallel load resonant converter in terms of the connecting relations between loads and resonant circuits. The working principle of the load resonant converters is that the current or voltage flowing through a switching element is reformed into a sinusoidal waveform through the resonance of resonant elements with loads; the switching elements are closed or opened at the zero-crossing of current or voltage; and a process of soft switching is achieved.
A Quasi Resonant Converter is so called because the time for its circuit working under resonance only is part of a switching period. A quasi resonant converter enables the current or voltage in a switching element to vary quasi-sinusoidally through resonance, thus to create a switching condition of zero current or zero voltage, which greatly reduces switching losses and switching noises of the converter.
A Multiple Resonant Converter is so called because there are not only one resonant topology and parameters in the circuit and it can achieve a switching tube's switching under zero voltage but only by the method of frequency controlling. A multiple resonant converter of zero voltage is usually used in practices mainly because it absorbs junction capacitor of a switching tube and a rectifier diode and achieves zero voltage switching of a switching tube and a rectifier diode.
A Zero Switching PWM Converter includes zero voltage PWM converter and zero current PWM converter which achieve PWM control by adding an auxiliary switching tube on the basis of a quasi resonant soft switch to control resonant process of resonant elements. Because only resonance is used for phase commutation and PWM working mode is still used after the phase commutation, the defect of hard switch PWM in the switching process is overcome and the advantage of low-steady-state losses and low-steady-state stress of hard switch PWM converter.
A Zero Transition PWM Converter includes ZVS-PWM converter and ZCS-PWM converter. Such a converter, combining a quasi resonant converter with a conventional PWM converter, interrupts a resonant process by an additional auxiliary active switch, causing a circuit to operate in ZCS or ZVS quasi-resonant mode in part of the time within a week and operate in PWM mode in the remaining part of the time within a week. It has both the characteristics of a soft switch and characteristics of PWM constant frequency duty ratio adjustment.
In ZVS-PWM converter and ZCS-PWM converter, a resonant inductor is cascaded in a main power return circuit, so there is a lot circulating energy in the circuit, which inevitably increases closing losses of a circuit. In addition, inductive energy storage is much related to input voltage and output load, which makes the soft switching condition of the circuit largely depend on the changes of the input power supply and output load.
The wide application of soft switching technology brings about revolutionary changes to the design of power electronics converter. The application enables the power electronics converter to have a higher efficiency—dramatic reduction of its own losses, to have a higher power density—dramatic decrease of its own size and weight, and to have a higher reliability. The application can also efficiently reduce the magnetic and environmental pollution caused by electric energy conversing devices and provide an efficient way and method for greatly developing green power electronics products.